CPU Scheduling

1
CPU Scheduling

• Basic Concepts

2
Chapter 5 CPU Scheduling

• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Thread Scheduling
• Multiple-Processor Scheduling
• Operating Systems Examples
• Algorithm Evaluation

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Objectives

• To introduce CPU scheduling, which is the basis
  for multiprogrammed operating systems
• To describe various CPU-scheduling algorithms
• To discuss evaluation criteria for selecting a
  CPU-scheduling algorithm for a particular system

4
Basic Concepts

• Maximum CPU utilization obtained with
  multiprogramming
• CPUI/O Burst Cycle Process execution consists
  of a cycle of CPU execution and I/O wait
• CPU burst distribution

5
Histogram of CPU-burst Times
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CPU-I/O Burst Cycle

• Process execution consists of a cycle of CPU
  execution and I/O wait
• Processes alternate between these two states
• Each scheduling decision is about which job to
  give to the CPU for use by its next CPU burst
• With time slicing, job may be forced to give up
  CPU before finishing current CPU burst

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CPU Scheduler

• Selects from among the processes in memory that
  are ready to execute, and allocates the CPU to
  one of them
• CPU scheduling decisions may take place when a
  process
• 1. Switches from running to waiting state
• 2. Switches from running to ready state
• 3. Switches from waiting to ready
• 4. Terminates
• Scheduling under 1 and 4 is nonpreemptive
• All other scheduling is preemptive

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Scheduling

• Managing resource (e.g., CPU)
• n processes
• A schedule is a listing
• Number of possible schedules if no
  interdependencies?
• Environment
• Number of processors involved
• Whether preemption is allowed (priorities or time
  slicing)
• Whether a process once active can become blocked
  (due to I/O, etc)

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Scheduling

• First the easy case
• One processor, no preemption, no blocking
• n/1 static scheduling problem
• n processes
• Know arrival times, processing times

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Scheduling

• Given the following information about process Pi
• Arrival Time ai
• Execution Time ei
• Due Time di
• We can obtain
• Completion (finish) Time fi
• Turn-around Time ti (fi ai)
• Waiting Time wi (ti ei)
• Lateness Li (fi di)
• Tardiness Ti max(0, Li)

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Example
Process P1 P2 P3 P4 P5 P6
pi 15 25 5 20 25 30
P4 P5 P3 P1 P6 P2
ti wi
P1 65 50
P2 120 95
P3 50 45
P4 20 0
P5 45 20
P6 95 65
0 20 45 50
65 95 120
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Scheduling Criteria

• CPU utilization
• keep the CPU as busy as possible
• Throughput
• of processes that complete their execution per
  time unit
• Turnaround time
• amount of time to execute a particular process
• Waiting time
• amount of time a process has been waiting in the
  ready queue
• Response time
• amount of time it takes from when a request was
  submitted until the first response is produced

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How good is a schedule? Performance Measures

• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time

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First-Come, First-Served (FCFS) Scheduling

• First-Come, First-Served (FCFS)
• Also First In, First Out (FIFO) or Run until
  done
• In early systems, FCFS meant one program
  scheduled until done (including I/O)
• Now, means keep CPU until process blocks

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First-Come, First-Served (FCFS) Scheduling

• Suppose that the processes arrive in the order
  P1 , P2 , P3 . Draw the Gantt Chart for the
  schedule and compute the average waiting time 3

Process P1 P2 P3
pi 24 3 3
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First-Come, First-Served (FCFS) Scheduling

• Suppose that the processes arrive in the order
  P2, P3, P1 . Draw the Gantt Chart for the
  schedule and compute the average waiting time 17

Process P1 P2 P3
pi 24 3 3
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First-Come, First-Served (FCFS) Scheduling

• There is a convoy effect as all the other
  processes wait for the one big process to get off
  the CPU
• Average waiting time for FCFS is often quite long
• What about average turnaround time?
• Pros and cons
• Simple ()
• Short jobs get stuck behind long ones (-)
• Analogy getting milk at a supermarket and
  waiting to pay, always stuck behind carts full of
  items. Upside get to read about space aliens!

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Example of (Shortest Job First) SJF

• Process Arrival Time Burst Time
• P1 0.0 6
• P2 2.0 8
• P3 4.0 7
• P4 5.0 3
• SJF scheduling chart
• Average waiting time (3 16 9 0) / 4 7

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SJF Scheduling example

• Sometimes called Shortest Processing Time (SPT)
  First
• Draw the Gantt Chart for a SPT schedule and
  compute the average waiting time and average
  turnaround time (assume all processes arrive at
  time 0)

Process P1 P2 P3 P4 P5 P6
pi 15 25 5 20 18 30
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Round Robin (RR)

• Each process gets a small unit of CPU time (time
  quantum), usually 10-100 milliseconds. After
  this time has elapsed, the process is preempted
  and added to the end of the ready queue.
• If there are n processes in the ready queue and
  the time quantum is q, then each process gets 1/n
  of the CPU time in chunks of at most q time units
  at once. No process waits more than (n-1)q time
  units.
• Performance
• q large ? FIFO
• q small ? q must be large with respect to context
  switch, otherwise overhead is too high